Zircon U-Pb Ages and Geochemical Characteristics of the McKinley Sequence and Associated Plutons, Central Alaska Range
Date Issued
2008
Date
2008
Author(s)
Hung, Chien-Hui
Abstract
Plutonic rocks in the Central Alaska Range were emplaced during importantectonic events including the end of Late Cretaceous-Paleocene arc magmatism, theerminal phase of terrane accretion, the Kula ridge subduction, regional strike-slipaulting, oroclinal rotation of western Alaska, and Eocene resumption of arcagmatism along the proto-Aleutian arc system. Here we report the first set ofA-ICPMS and SHRIMP zircon U-Pb ages for granitoids from (1) the McKinleyequence, (2) the Composite Plutons and (3) the Foraker Pluton, central Alaska Range.wo peraluminous granites from the McKinley Sequence yielded 206Pb/238U ages of2±1 and 60±1 Ma; an A-type granite from the same area gave age of 64±1 Ma. Tworanodiorites from the Composite Plutons and a granite from the Foraker Pluton gaveges of 69±1, 67±1 and 37±1 Ma, respectively.he age results indicate that the Composite Plutons and the McKinley Sequenceranites are older than previously reported and that the McKinley granites can beivided into two magma suites that consist of peraluminous and A-type compositionsccurring at ca. 62-60 and 64 Ma, respectively. Nd and Sr isotope data suggest aemporal change in the magma’s isotopic compositions, with the Foraker graniteshowing higher and more heterogeneous εNd(T) values that range from about +6 to -2nd the Composite and McKinley Sequence granitoids showing lower and lesseterogeneous εNd(T) values from +1 to -3. Besides, zircon Hf isotopic data of theated samples show that a granite from the Foraker plutons has the highest zirconHf(T) values from +15 to +10, in contrast to the Composite plutons that showpparently lower zircon εHf(T) values from +4 to -3. Peralumous and A-type granitesrom the McKinley Sequence have zircon εHf(T) values from +2 to -5 and +8 to +1,espectively.he Composite and McKinley Sequence plutonic rocks are interpreted as productsf the ending phase of Late Cretaceous-Paleocene arc magmatism. The peraluminousranites from the McKinley sequence may have been sourced from the remnantantle wedge, previously responsible for generating the Composite plutons andarlier arc magmas. Besides, generation of the A-type granites can be attributed to theroclinal rotation of western Alaska and ridge subduction that started around 64 Ma, aechanism that may also have resulted in broadly coeval volcanism in the nearbyantwell basin and northern Talkeetna Mountains. The Foraker pluton are interpreteds the product of renewed magmatism along the proto-Aleutian arc, in response toorth or northwestward subduction of the Pacific plate during 45-30 Ma.
Subjects
Alaska Range
the McKinley Sequence
zircon U-Pb dating
Sr-Nd isotopes
Hf isotope
Type
thesis
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